1. Field of the Invention
The present invention relates to quantization functional device utilizing a resonance tunneling effect and a method for producing the same. In particular, the present invention relates to a resonance tunneling diode, a method for producing the same, a hot electron transistor, and a method for producing the same.
2. Description of the Related Art
Recently, quantization functional devices utilizing a quantum effect have been actively studied. One type of quantization functional device which has been proposed for practical use is a device utilizing the resonance tunneling effect of electrons, for example, a resonance tunneling diode.
Such a device requires a double barrier structure in which a quantum well having a size approximately identical with the scattering length of the electrons and tunnel barriers sandwiching the quantum well are provided. One type of such a double barrier structure utilizes a semiconductor heterojunction, which is generally realized by way of crystal growth of thin films, each formed of a compound semiconductor material and having several atom layers (see, for example, Reona Ezaki and Hiroyuki Sakaki, Super-lattice Hetero Structure Device, published by Kogyo Chosakai, pp. 397 to 435 (1988)).
Another type of double barrier structure which has been proposed uses silicon materials, and is produced by forming a silicon oxide film and a polysilicon film on a silicon substrate (see, for example, Saki et al., Resonant Tunneling through SiO.sub.2 /Si/SiO.sub.2 Double Barriers, Extended Abstracts of the 52nd Autumn Meeting of the Japan Society of Applied Physics, No. 2, pp. 653, 10a-B-3, 1991).
Briefly referring to FIGS. 1A through 1D, a conventional resonance tunneling diode utilizing compound semiconductor materials is produced in the following manner.
Compound semiconductor materials are laminated by MBE (molecular beam epitaxy).
As illustrated in FIG. 1A, on a first Si-doped GaAs layer 11, a first AlGaAs layer 12 is grown in a thickness of 2.3 nm. Then, on the first AlGaAs layer 12, a GaAs layer 13 (FIG. 1B) having a thickness of 7.0 nm, a second AlGaAs layer 14 (FIG. 1C) having a thickness of 2.3 nm, and a second Si-doped GaAs layer 15 (FIG. 1D) are sequentially laminated. As a result, a resonance tunneling diode having a double barrier structure including the first AlGaAs layer 12/the GaAs layer 13/the second AlGaAs layer 14 is produced.
With reference to FIGS. 2A through 2E, another conventional resonance tunneling diode utilizing silicon materials is formed in the following manner.
An n-type silicon substrate 21 as illustrated in FIG. 2A is prepared. By dry oxidation at a temperature of 1,000.degree. C., a first silicon oxide film 22 is formed at a thickness of 3 to 4 nm on the silicon substrate 21 (FIG. 2B). On the silicon oxide film 22, a polysilicon layer 23 is formed at a thickness of 8 to 12 nm by LPCVD (low pressure chemical vapor deposition) (FIG. 2C). By dry oxidation at a temperature of 1,000.degree. C., a second silicon oxide film 24 is formed at a thickness of 3 to 4 nm on the polysilicon layer 23 (FIG. 2D). By depositing aluminum in a vacuum, an aluminum electrode 25 is formed on the second silicon oxide film 24 (FIG. 2E). In this manner, a resonance tunneling diode having a double barrier structure including the first silicon oxide film 22/the polysilicon layer 23/the second silicon oxide film 24 is produced.
Another type of quantization functional device utilizing a resonance tunneling effect is a resonance tunneling transistor or a hot electron transistor (referred to as a hot electron transistor hereinafter). In a hot electron transistor, another electrode for directly controlling the level of voltage applied to the quantum well is provided. Thus, the electrodes at the two ends of the double barrier structure respectively act as a collector and an emitter, and the quantum well acts as a base. In such a configuration, hot electrons from the emitter pass through the base, which is a thin layer and reach the collector. The hot electron transistor uses, for example, CoSi.sub.2 /CaF.sub.2 or the like as described in J. C. Hensel et al., Appl. Phys. Lett., vol. 47, pp. 151 (1985).
The above-described conventional quantization functional devices have the following problems.
In devices using compound semiconductor materials, electrons are not sufficiently confined in the quantum well due to a low height of the tunnel barrier (1.5 eV or less). As a result, even if the electrons in the quantum well are not in a resonance state, some of the electrons pass through the double barrier structure. Accordingly, the V-I characteristic of the device does not have a high P/V ratio (ratio of the peak current relative to the valley current).
In devices using silicon materials, it is difficult to form a quantum well having satisfactory crystallinity. This deteriorates the negative resistance characteristics of the devices.
Production of conventional hot electron transistors requires new facilities and technologies for handing materials such as CoSi.sub.2 and CaF.sub.2, in addition to the existing facilities and technologies for mainly handling silicon materials. In order to mass produce the hot electron transistors, problems concerning production efficiency and production cost need to be solved.